U.S. patent application number 12/524016 was filed with the patent office on 2010-05-13 for etching solution and etching method.
This patent application is currently assigned to GP SOLAR GMBH. Invention is credited to Peter Fath, Ihor Melnyk.
Application Number | 20100120248 12/524016 |
Document ID | / |
Family ID | 39530927 |
Filed Date | 2010-05-13 |
United States Patent
Application |
20100120248 |
Kind Code |
A1 |
Fath; Peter ; et
al. |
May 13, 2010 |
ETCHING SOLUTION AND ETCHING METHOD
Abstract
An etching solution contains water, nitric acid, hydrofluoric
acid, and sulphuric acid. More specifically it contains 15 to 40%
by weight of nitric acid, 10 to 41% by weight of sulphuric acid and
0.8 to 2.0% by weight of hydrofluoric acid. The etching solution is
used for etching silicon and to etching methods for silicon
wafers.
Inventors: |
Fath; Peter; (Konstanz,
DE) ; Melnyk; Ihor; (Konstanz, DE) |
Correspondence
Address: |
LERNER GREENBERG STEMER LLP
P O BOX 2480
HOLLYWOOD
FL
33022-2480
US
|
Assignee: |
GP SOLAR GMBH
Konstanz
DE
|
Family ID: |
39530927 |
Appl. No.: |
12/524016 |
Filed: |
January 22, 2008 |
PCT Filed: |
January 22, 2008 |
PCT NO: |
PCT/DE08/00099 |
371 Date: |
January 20, 2010 |
Current U.S.
Class: |
438/692 ;
252/79.2; 257/E21.483 |
Current CPC
Class: |
H01L 21/30604 20130101;
H01L 21/31111 20130101; H01L 21/02087 20130101; H01L 21/0209
20130101; C09K 3/1463 20130101 |
Class at
Publication: |
438/692 ;
252/79.2; 257/E21.483 |
International
Class: |
C09K 13/04 20060101
C09K013/04; H01L 21/461 20060101 H01L021/461 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 22, 2007 |
DE |
10 2007 004 060.3 |
Claims
1-22. (canceled)
23. An etching solution, comprising: 15 to 40 percent by weight of
nitric acid; 10 to 41 percent by weight of sulfuric acid; 0.8 to
2.0 percent by weight of hydrofluoric acid; and remainder
water.
24. The etching solution according to claim 23, wherein: said
nitric acid is 20 to 30 percent by weight; said sulfuric acid is 18
to 35 percent by weight; and said hydrofluoric acid is 1.0 to 1.7
percent by weight.
25. The etching solution according to claim 23, wherein: 27 percent
by weight of said nitric acid; 26 percent by weight of said
sulfuric acid; and 1.4 percent by weight of said hydrofluoric
acid.
26. The etching solution according to claim 23, wherein said water
is at least partly deionized.
27. An etching method, which comprises the steps of: providing an
etching solution containing 15 to 40 percent by weight of nitric
acid, 10 to 41 percent by weight of sulfuric acid, 0.8 to 2.0
percent by weight of hydrofluoric acid, and remainder water; and
etching silicon, including doped silicon, silicon-containing
compounds, and silicate glasses with the etching solution.
28. The method according to claim 27, which further comprises
etching one of phospho- and borosilicate glass with the etching
solution.
29. The method according to claim 27, which further comprises using
the etching solution for at least partly overetching silicon
wafers, including doped silicon wafers.
30. The method according to claim 29, which further comprises using
the etching solution for etching silicon-containing compounds,
including silicate glasses, present on the silicon wafers.
31. The method according to claim 29, wherein the silicon wafers
have at least partly a surface structuring including mechanical
structurings or chemical surface structurings.
32. The method according to claim 29, which further comprises using
the etching solution for overetching edge areas of the silicon
wafers.
33. The method according to claim 29, wherein the silicon wafers
are for producing solar cells into which a dopant has been
indiffused, and the etching solution is used for at last partly
removing regions doped by the diffusion, including for insulating
edge areas of the silicon wafers.
34. An etching method for silicon wafers for partly removing
silicon, including doped silicon, silicon-containing compounds, and
silicate glasses, which comprises the step of: bringing at least
one part of each individual silicon wafer into contact with an
etching solution, the etching solution is held at a temperature of
between 4.degree. C. and 15.degree. C., the etching solution
containing: 15 to 40 percent by weight of nitric acid; 10 to 41
percent by weight of sulfuric acid; 0.8 to 2.0 percent by weight of
hydrofluoric acid; and remainder water.
35. The etching method according to claim 34, which further
comprises partly dipping the silicon wafers into the etching
solution.
36. The etching method according to claim 35, which further
comprises partly dipping the silicon wafers having a surface
structure into the etching solution such that the surface structure
lies above a liquid level of the etching solution.
37. The etching method according to claim 36, which further
comprises moving the silicon wafers in a dipped position parallel
to a surface of the etching solution.
38. The etching method according to claim 37, which further
comprises moving the silicon wafers on one of rollers and conveyor
belts parallel to the surface of the etching solution.
39. The etching method according to claim 35, which further
comprises dipping the silicon wafers into the etching solution such
that of two sides of each silicon wafer which have a largest area,
one is situated below a liquid level of the etching solution and
one is situated above the liquid level of the etching solution.
40. The etching method according to claim 39, which further
comprises dipping the silicon wafers into the etching solution such
that each of edge areas of the silicon wafer, along a longitudinal
extension direction thereof, are always situated at least partly
below the liquid level of the etching solution.
41. The etching method according to claim 34, which further
comprises holding the etching solution at a temperature of between
7.degree. C. and 10.degree. C.
42. The etching method according to claim 34, which further
comprises etching the silicon wafers into which a dopant has been
indiffused and doped regions of the silicon wafers are at least
partly removed during the etching.
43. The etching method according to claim 34, which further
comprises etching the silicon wafers into which a dopant has been
indiffused and silicate glasses, including phospho- or borosilicate
glasses, formed during diffusion are at least partly removed during
the etching.
44. The etching method according to claim 34, which further
comprises etching the silicon wafers in a continuous method.
Description
[0001] The invention relates to an etching solution in accordance
with the preamble of claim 1, to the use of said etching solution
for etching silicon, and to an etching method in accordance with
the preamble of claim 12.
[0002] Semiconductor components play a major part in many branches
of technology. In accordance with the diversity of different
components, a wide variety of requirements are made of the
technologies for processing this material. Among these, etching
technologies and etching methods have acquired great importance.
This is due to the fact that, with the aid thereof, firstly the
material can be selectively processed at individual locations, and
secondly it is possible to process large numbers, in particular on
an industrial scale. Most semiconductor components fabricated at
the present time are based on silicon as starting material.
[0003] During the selective processing of individual locations of
the components or blanks it must be ensured that the etching
solution only reaches those locations at which material is intended
to be removed, but other regions will be unaffected. This is
usually done by regions that are not to be etched being covered,
masked as it were, with a material that is resistant to the etching
solution. Such masking can be effected by applying
etching-solution-resistant resists, films, sheets or the like. Such
maskings are complicated. If possible, therefore, recourse is had
to other effects in order to protect individual regions from
contact with the etching solution, for example to wetting phenomena
or the effect of gravitation. In the simplest case, a blank is only
partly held into an etching solution that does not completely wet
it, with the result that the blank is etched below the liquid level
of the etching solution and below the wetted regions of the blank,
but is not etched above the wetted regions.
[0004] The extent to which a selective processing of individual
regions can be achieved solely by partly dipping the blank into the
etching solution, without other regions being detrimentally
affected, depends on the individual case. In particular, surface
structures, on account of capillary effects, can have the
consequence that etching solution reaches regions at which no
etching process is intended.
[0005] Therefore, the present invention is based on the object of
providing an improved etching solution which enables more precise
selective processing of individual regions.
[0006] This object is achieved according to the invention by means
of an etching solution comprising the features of claim 1.
[0007] Furthermore, the invention is based on the object of
improving the etching of silicon, in particular of silicon wafers
with a surface structuring.
[0008] This object is achieved by means of the use according to the
invention of the etching solution in accordance with claim 5.
[0009] Moreover, the invention is based on the object of providing
an improved etching method for silicon wafers.
[0010] This object is achieved by means of an etching method
comprising the features of claim 12.
[0011] Dependent subclaims respectively relate to advantageous
developments.
[0012] The etching solution according to the invention has a
comparatively low surface tension in conjunction with a good
etching effect in the case of inorganic materials, in particular in
the case of silicon. Consequently, it has a reduced tendency to
penetrate into small-dimensioned surface structures. Such surface
structures can be formed by microcracks or processing structures in
the surface of the blank to be etched. Moreover, it is also
possible in part to introduce surface structurings--often also
referred to as surface texturings--into the blank. Such surface
structurings can be introduced mechanically, for example, such as
occurs in particular during the mechanical structuring of solar
cells for the purpose of increasing the coupling-in of light.
However, they can also be the consequence of a preceding etching
process. By way of example, anisotropic etching solutions are used
in turn in the field of solar cell fabrication, which etching
solutions have etching effects of different magnitudes in different
spatial directions, if appropriate depending on the crystal
orientation of a crystal to be etched, with the result that a
surface structure is formed. Said surface structure can in turn
bring about an increased coupling-in of light into the solar
cells.
[0013] Penetration of the etching solution into said structures
would damage the surface structuring. This risk is significantly
reduced, however, in the case of the etching solution according to
the invention. Consequently, the regions without surface
structuring can be brought into contact with the etching solution
and etched without the regions with a surface structure being
damaged in the process.
[0014] Preferably, the etching solution according to the invention
is used for, if appropriate selectively, etching silicon or
silicon-containing compounds, in particular silicate glasses. This
should also be understood to include doped silicon. Moreover, an
application in the field of other non-organic materials, in
particular semiconductor materials, is also conceivable.
[0015] In the case of etching silicon, but also other semiconductor
materials, the sulfuric acid in the etching solution according to
the invention does not participate in the chemical etching
reaction. It primarily serves to increase the specific density of
the etching solution. As a result of the chemical reactions
proceeding during the etching process and the associated conversion
of the reagents, although the specific density of the etching
solution decreases per se, this is approximately compensated for by
the etched-away silicon now situated in the etching solution.
Consequently, it is not necessary to supply sulfuric acid for
maintaining the initial specific density.
[0016] The etching solution according to the invention can
advantageously be used in particular in the field of silicon
semiconductor technology. In this branch of technology, dopants are
indiffused into silicon wafers, with silicate glasses being formed,
which often have to be removed. This can be effected by means of
the etching solution according to the invention. Boro- or
phosphosilicate glasses produced during phosphorus or boron
diffusions can be removed, inter alia. In addition, doped layers
can be removed locally with at the same time a low risk of damage
for the surrounding doped regions.
[0017] In the abovementioned branch of silicon semiconductor
technology, silicon wafers are usually used as starting material
for the production of the semiconductor components such as
integrated circuits or solar cells. They are largely produced by
sawing cast silicon blocks into wafers or sawing off wafers from
pulled silicon columns. During these sawing processes, which are
usually carried out by means of wire saws, the surface of the
silicon wafers is damaged. This is normally removed by overetching
the silicon wafers, in which case the etching solution according to
the invention can likewise be used.
[0018] In addition, in other methods, silicon wafers are pulled
from a silicon melt directly with the desired thickness. These
silicon wafers are often referred to as silicon ribbons. In the
case of the latter, although sawing damage in the sense explained
is not present, the layer near the surface is often relatively
highly contaminated, with the result that an overetching of the
silicon wafers is performed here for the purpose of at least partly
removing these contaminated layers. The etching solution according
to the invention can once again be employed in this case.
[0019] The invention is explained in more detail below on the basis
of an exemplary embodiment illustrated in figures, in which:
[0020] FIG. 1 shows a schematic illustration of a silicon wafer
provided with a surface structuring in an etching solution
according to the invention during the etching according to an
etching method according to the invention in a side view.
[0021] FIG. 2 shows a front view of the silicon wafer from FIG.
1.
[0022] FIG. 1 shows a silicon wafer 3 provided for fabricating a
solar cell, which silicon wafer has already been subjected to a
phosphorus diffusion. Consequently, it bears a phosphorus-doped
layer and a phosphosilicate glass over its entire surface.
Furthermore, the silicon wafer was provided with a surface
structuring 5 prior to the phosphorus diffusion. Said surface
structuring was introduced mechanically in the present case.
However, the way in which the surface structure is introduced is
unimportant for the invention. This can for example also be
effected by chemical methods such as anisotropic etching methods or
etching methods that act in a manner dependent on crystal
orientation.
[0023] The two side areas of the silicon wafer 3 that have the
largest area form the front side 25 and the rear side 27. In
addition, the silicon wafer 3 peripherally has edge areas 7, 9, of
which the edge area 7 can be seen in FIG. 1. Each of the edge areas
has a longitudinal extension 8 or 10, respectively.
[0024] The silicon wafer 3 is partly dipped into an etching
solution 1. In this case, the dipping depth is chosen such that
each edge area, in particular the edge areas and 9, along the
direction of the longitudinal extension thereof, along the
direction of the longitudinal extensions 8 and 10 in the case of
the edge areas 7 and 9, are always situated partly below the liquid
level 11 of the etching solution. In this way it is possible to
remove the phosphosilicate glass and the phosphorus-doped layer
situated underneath at the edge areas such that when a conductive
layer is applied to the rear side 27 of the solar cell, there is no
electrically conductive connection to the front side via the edge
areas, which would short-circuit the solar cell. Moreover, it is
possible to remove the phosphorus-doped layer and the phosphorus
glass on the rear side 27.
[0025] The etching solution used is an etching solution 1 according
to the invention comprising water, nitric acid, hydrofluoric acid
and sulfuric acid, which contains 15 to 40 percent by weight of
nitric acid, 10 to 41 percent by weight of sulfuric acid and 0.8 to
2.0 percent by weight of hydrofluoric acid. An etching solution 1
containing 27 percent by weight of nitric acid, 26 percent by
weight of sulfuric acid and 1.4 percent by weight of hydrofluoric
acid is preferably used. Furthermore, deionized water is used in
the present case in order to prevent an introduction of
contamination into the silicon wafer 3, which could impair the
performance of the finished solar cell. With less stringent purity
requirements, water in a generally available form can be used
instead.
[0026] During the etching the etching solution 1 is always held at
a temperature of between 4.degree. C. and 15.degree. C., preferably
at a temperature of between 7.degree. C. and 10.degree. C. This
makes it possible, in conjunction with the etching solution 1
according to the invention, for said etching solution not to pass
into and damage parts of the surface structuring 5 on account of
capillary effects (explained above).
[0027] This is of importance particularly when thin silicon wafers
3 are intended to be etched. Otherwise, it is possible to maintain
enough distance between the liquid level 11 of the etching solution
1 and the lower edge of the surface structuring 5 during the
etching process, such that the risk of damage to the surface
structuring is low. However, semiconductor components are normally
made thin. In the case of solar cells for example, the thickness,
that is to say the distance between front side 25 and rear side 27
of the silicon wafer 3 is usually in the range of 100 nm to 350 nm
with a trend to more extensive reduction of the thickness. In these
thickness ranges it is of crucial importance to prevent penetration
of etching solution 1 into the surface structuring 5 by capillary
effects, since the liquid level 11 of necessity is situated only
slightly below the lower edge of the surface structuring 5, if it
is to be ensured that the edge areas 7 and 9, along the direction
of the longitudinal extensions 8 and 10, are always situated partly
below the liquid level 11 of the etching solution 1.
[0028] Such restrictions are also found in the production of other
semiconductor components, in particular those composed of silicon
such as silicon-based integrated circuits or nanomachines such as
e.g. nanomotors or nanopumps. Consequently, the invention can
likewise be used beneficially there.
[0029] In the exemplary embodiment in FIGS. 1 and 2, the dipping
depth of the silicon wafer is determined by the conveyor belts 13
and 15 which are illustrated in FIGS. 1 and 2 and on which the
silicon wafer 3 bears. Instead of the latter, other devices on
which the silicon wafer 3 bears, for example lowerable wire grids
or the like, are also conceivable, of course. The advantage of the
conveyor belts 13 and 15, the number of which can be chosen as
desired, in principle, depending on the mechanical properties of
the silicon wafer 3, is that they can be driven comparatively
simply, by means of drive rollers 17, 19, 21 for example. This
enables silicon wafers 3 to be etched in an efficient continuous
method. The silicon wafers 3 are placed onto the driven conveyor
belts 13 and 15 and transported through the etching solution at a
defined dipping depth parallel to the surface 2 of said etching
solution before they are fed to further process units.
[0030] Instead of conveyor belts 13, 15, it is also possible, in a
known manner, to provide transport rollers which are arranged in a
continuous sequence and which transport the silicon wafer through
the etching solution 1 and enable a continuous method. In the case
of semiconductor components that can be subjected to less
mechanical loading, however, conveyor belts that are elastic to a
certain extent may be more advantageous.
LIST OF REFERENCE SYMBOLS
[0031] 1 Etching solution [0032] 2 Surface of etching solution
[0033] 3 Phosphorus-doped silicon wafer with phosphosilicate glass
[0034] 5 Surface structuring [0035] 7 Edge area [0036] 8
Longitudinal extension of edge area [0037] 9 Edge area [0038] 10
Longitudinal extension of edge area [0039] 11 Liquid level of
etching solution [0040] 13 Conveyor belt [0041] 15 Conveyor belt
[0042] 17 Drive roller [0043] 19 Drive roller [0044] 21 Drive
roller [0045] 25 Front side of silicon wafer [0046] 27 Rear side of
silicon wafer [0047] 30 Direction of movement of the silicon
wafer
* * * * *